1. Field of the Invention
The present invention relates to a precipitated silica surface-modified by treatment with a polymer, and to a process for its preparation.
2. Description of the Related Art
The surface modification of precipitated silicas has long been known and serves for producing functional-enhancement materials for silicone rubbers, ink-jet applications or coating materials (as matting agents).
Thus EP 0 922 671 describes the preparation of a hydrophobic precipitated silica where a polyethylene wax emulsion is added to a conventional, dried precipitated silica which is subsequently ground in a classifier mill or opposed-jet fluid-bed mill.
DE 25 13 608 describes a process for hydrophobicizing finely divided silicas by treating the dry silica with organosilanes in a fluidized state at temperatures from 200 to 300° C.
To simplify the hydrophobicization process EP 0 341 383 proposes introducing silicone oil emulsions into a suspension of precipitated silicas.
A similar process is described by DE 24 35 860, in which a suspension of precipitated silica is reacted with optionally precondensed organohalosilanes. After the solid has been isolated by filtration it is heated at temperatures from 300 to 400° C. and subsequently ground.
Precipitated silicas are frequently used for the matting of coating surfaces. A coating film thus matted ought to exhibit minimal gloss at a variety of viewing angles. Since hydrophilic precipitated silicas may tend toward severe sedimentation in coating materials and therefore are difficult if not impossible to reagitate, it is common to use wax-coated precipitated silicas as matting agents. The wax-coating of the silica surface substantially improves the sedimentation behavior. If the matting agent sediments at all, such sedimentation is so minimal that it can be reagitated without expending much energy.
Where clear coating materials are used it is desirable that the transparency of the coating material not be influenced by the incorporation of the matting agent.